As stated by Albert Einstein’s theory of Special Relativity, the universe too has a speed limit. The speed of light (speed of light because this speed was first discovered in visible light) that has a speed of 300,000 kilometers per second or 186,000 miles per second has set the cosmic speed limit. No object with mass can travel at the speed of light. We have tried to exceed the speed of light at LHC, but we could not do so). To travel at the speed of light, particles have to be massless (like photons). But, is this the maximum speed the universe allows for, or is there something that can exceed that threshold?
The tachyons, first theorized by Gerald Feinberg in 1967 is a particle made up of negative mass that travels faster than light. As a positive mass (stars, atoms, you and me) always travels below the speed of light and massless particles always travel at the speed of light, these tachyons with negative mass always travel faster than light. Particles with mass can never exceed the speed of light because when they approach those speeds, their mass starts to approach infinity. Due to this, an infinite amount of energy will be required to further increase the speed to light speed. But, these tachyons can never travel below the speed of light because unlike normal matter, when the speed of such an object is reduced, its energy tends to approach infinity and hence to reduce its speed to exactly light speed will require infinite energy. Even if we are able to slow tachyons down to light speed, these particles will cease to exist.
Experience Of Time
Another interesting phenomenon that special relativity allows for is time dilation. The faster you move through the dimensions of space, the slower you move through time. So time will pass very slowly for a particle with positive mass that is approaching light speed. Massless particles such as photons do not experience time. They are the same age today as they were at the time of their creation during the big bang. Tachyons have been predicted to experience time in backward direction. They are reverse aging and travelling towards the past. Thus, you can also use these particles to send messages in the past.
Possibility Of Existence
However, the existence of this particle is highly unlikely (even theoretically) due to tachyon condensation and the emergence of new paradoxes. Tachyon condensation will cause tachyons to decay into particles in order to reach a vacuum state (highest stability), thus preventing tachyons from existing. The existence of these particles also lead to interesting paradoxes such as the tachyonic anti-telephone paradox where you send a message to your past self in order to correct your mistake, but if you never made any mistake, why would you send a message into the past in the first place. Another reason why these particles can’t exist is because they will lose their energy in a phenomenon known as Cherenkov radiation.